AIMS: Hepatitis C Virus (HCV) harms a substantial number of Veterans Administration (VA) patients, and therapy is complicated by many different strains of the virus. We propose to study the role of viral genetics in response to therapy, particularly in how HCV responds to antivirals called cyclophilin inhibitors that have activity against HCV and HIV. We will determine how changes in HCV genetics that happen in patients alter cellular and biochemical tests that may predict response to cyclophilin inhibitors. We will also pilot the first catalog of important viral strains in VA patients with HCV. RESEARCH DESIGN &METHODS: We will capitalize on a large HCV-infected liver transplant population that have pre- and post-transplant samples to determine if the earliest version of a cyclophilin inhibitor (cyclosporine) that is already in use in transplant patients for purposes other than as an antiviral breeds HCV resistance. We will use cell-free studies to determine how cellular cyclophilin proteins interact with two viral proteins NS5A and NS5B and how viral protein interactions differ depending upon whether they are from strains of HCV that respond to cyclophilin inhibitors or not. We will also generate a pilot HCV genetic database. The database will catalog important viral genetic variation from strains in the VA and how they change over time. Whether or not our results ultimately lead to the intentional use of cyclophilin inhibitors to treat HCV, this research will reveal fundamental insights into the regulatory viral protein called nonstructural protein NS5A, which is the target of not only cyclophilin inhibitors, but also of other drugs in preclinical development. HEALTH RELATEDNESS: HCV is the major cause of liver disease and liver cancer in the VA and the US in general. By facilitating the cure of HCV, we can prevent the complications of liver cancer and liver disease in VA patients. To do this, not only are new antivirals needed, but also new treatment strategies that encourage greater individualization of care. HCV has an extremely high genetic diversity. That level of viral diversity makes it highly likely that anti-HCV drugs will be more active against some HCV strains than others. In other words, the risk-benefit ratio of each HCV treatment is likely influenced by the genetics of the specific strain. Recently, several investigational and even some FDA-approved drugs have been shown to have significant anti-HCV activity. But in most, if not all, cases, the relationship between the drug's potency and the genetic sequence of different HCV strains is largely undetermined. All of the viral drug targets interact with the viral regulatory protein NS5A, which is itself also a drug target. These studies will shed insight into the potential utility of cyclophilin inhibitors to treat VA HIV/HCV co-infected patients as well as HCV mono-infected patients. How cyclophilin inhibitor antivirals or other small molecule inhibitors should be optimally paired together in antiviral cocktails is a crucial decision that requires VA-specific data, in addition to industry-sponsored trials outside the VA in order to lessen the rising HCV burden of disease. PUBLIC HEALTH RELEVANCE: The VA currently cares for ~210,000 patients with chronic HCV viremia, the largest population of HCV patients in any one US health care system. Due to HCV, the rate of liver cancer in the VA increased by almost a third recently. HCV is also responsible for ~ 10% of NonHodgkins lymphoma in the VA and is associated with diseases outside the liver. Research from the VA's Center for Quality Management in Public Health among other VA sites shows both that many VA patients are not considered candidates for current therapy due to drug toxicities, and that therapeutic results in veterans are disappointing relative to industry-sponsored trials. Many veterans, particularly Vietnam-era veterans contracted HCV due to transfusions during combat duty. The cost of caring for HCV is also substantial with an estimate of over $700 million annually nationwide. Due to the high cost of care for patients with end-stage-liver disease, the cost associated with HCV is expected to rise over the next decade. Therefore if new, more predictably effective therapy can be offered, the rising cost of HCV may be reversed, similar to what new drug combinations did for the cost of HIV care. This study explores how a new class of antivirals -- cyclophilin inhibitors -- acts on HCV, with a focus on how viral genetics influences efficacy. We also propose to develop the first HCV viral genetic database for the VA that will give insight into how HCV genetics impacts treatment response. This database will catalog VA strains of HCV that need to be targeted by antivirals like cyclophilin inhibitors as well as others that are currently being investigated mostly outside the VA.